Radio and telegraph receiver



D. A. GRIFFIN RADIO AND TELEGRAPH RECEIVER Jan. 9, y1940.

Filed June 16, 193'] INVENTOR km @www fda ATTORNEY Patented llan. 9, l

nana A.' Griffin, rr'vingtonf,fN. J.; assignee@ tector which is .supplied withoscillatingv current l. by thehighfreque'ncy oscillator, all inya manner rwell known` inthe art.` The amplifier, detector My invention relates `to radio and teleglhie receivers and more particularly yto circuits; for

reducing noise interference such as'static in these receivers. i l 1l I am aware of 'a numb r` of circuits proposed heretofore for reducing noise interference by limiting the amplitude of noise pulses `to approximately thatof the signal voltage beingreceived,l These circuits lhave been devised primarily .,f or

telephone reception and while they havegsome value in the reception oftelegraphsignalsthey mit the transmission of noise pulses that are 1 lo Adonot produceoptimum results. One of theprin'- jcipal reasons for their deflciencyis that'theyper-v lbelowithe amplitude of the Acarrier wave,` :This

results in` an audible receptionof a `certain n ao i efiicientreception of radioand telegraphicsig-l,

amount vof objectionable 'noise vparticularly dur.- i

ingrthe yinter-character spaces of telegraph 4sig'- nals. Y

'I'he system of my invention 'overcomes many of thedisadvantages and limitations of. the prior art systems such as those mentioned above. It comprises a novel form of-demodulator circuit that acts as a gate and effectively limits noise pulses which are in excessfiof ythe carrier ampli, z.

tude, and in combination therewith a tone` gen-v erating circuit that will not respond tonoise voltages` whose average energy content iS' appreciably -below that of the carrier Wave being \.re. ceived. Accordingly, my system provides. for` the nals without any, undesirable.interferencev Vby of my invention a diode Vacuum tube is used in static orlother noises regardless oftheirampli4` tude. i t i In accordance with' one preferred embodiment the receiver to decrease the detecting efficiency' Aper-heterodyne radio receiver. j e i Referring nowto the drawing,v the signal picked up by the .antenna system, I.f2, is ampli# of the detector when noise pulses greater than the 'carrier are received and thereby eliminates these noises,V and a gaseous discharge tube is j' used to eliminate noise pulses of lesser'arnplitude than thek carrier. .l i

The novel features of my invention are set forth with particularity' in rthe appended claims; The invention itself, however, both as toits organization and method of operation will be easilyunderstood from the following description taken in conjunction with the` accompanying. drawing which shows onel illustrative ,l and non-limiting embodiment of my invention [as applied to la sufied by the radio` frequency amplifier andis con-- verted to intermediate frequency in the first rdeand oscillator. are of conventionaltype and-are e therefore shown ,.,diagramrnaticallm .'With identifying'legends.. y

TThe. signal. converted?tointermediate free l quency, traversesy primary 'winding .5f r*of the inl-v termediatejfrequency transformer 3 [and induces a corresponding voltage in thesecondarywinding 6.1I3oth primary andfvsecondary windingsr 5l and 1;; 6 `are tunedby variable vcondensers 4 and 1, rel spectively. One endfdf secondary winding 6 is connected to y.the control grid 8 of atetrode yscreen .y `grid type of'vacuu'mtube VThe; other end 01.'.16` #l winding. 6 .is 'groundedfr .The cathode-I0 vof the :.:f

vacuum'tube-Sis connected to a potentiometer II which is by-passed by'a condenserrlly The po- ,j tentiometer..;I I .servesfl'as an intermediate frequency I gain l .controh The output circuit ofvtube Bl'iscomplete'd from .Y the anode I3 `through.,tl'e .primary winding I4 of thefintermediate frequency transformer |57- to the-positive terminalof. a Bbattery.` I9; The heati er element Han'd the'screenfgrids.' Gl, vof tube 9 are. provided witl'nnecessary heater and screen potentials fin the :customary manner, the elec#- trical connectionsto ythe Ivoltage supplyr being omitted for salse .of simplicity.I The intermediate frequency outputicurrent of `tube 9 flowing from the transformer] 5 tothelintermediate frequency.

a resistance and acondenser 2I. constantcircuit is connected also `to the anode 22` .of aldiode tube 23 and to the grid 24 of a detector ,l j

triode tubegZE. I'fIIIhe cathode 40` of diode tube..23 is connectedtoa potentiometer llll, which-is connected inshunt 4with 'al bias battery 412. A con-Y denser 43 4connects this cathode to ground. Heat# the heaters H 4of .diode tube 23 and triode tube 25.

The anode 36 `of I detector `tube.25. isi-connectedn to a .resistanceZ'I and vtoa switch28'wlnchy may be closed, by moving upward` to a contact 29 when radio signals. are beingreceived, or` moving down-Lx i ward to a contact 30 when reception. of telegraphT ic signals is'deSired.` .Resistance islshunted bya condenser 34 when switch 28 is closed down-1 wardfto contact' 30. This resistancefz'lv is conv` '45 lercrurrent.- ishsupplied by means, not shown, l.to

zicl

nectedat its lower end to the plus terminal of the B battery i9. The cathode 31 of detector tube 25 is connected to ground through cathode resistor 33 which is shunted by a condenser 39.

A telephone receiver 3| is connected` in `shunt with resistance 21, when switch 2S is pushed up' to contact 20. When switch 28 is on contact 30` a connection iseStabIished` through resistance 32 to a discharge tube 33. I have found a GeneralElectric half watt neon lamp tobe satisfactory fortube `33. condenser 35 in series with a telephone receiver 36. I

Oi" a radio-telegraphic receiverl system embodying some ofl the novel features"of'my'inventionl' The operation of this system, with particular ref'- erence to thefeatures of my present invention Will now beldescribed."

The triode tube 25 operates as'a'biased detector, its cathode resistor 3B being'used to' bias-the grid 24 ldownen the lower knee of the tubes charac- "i teristic curve, the value of` bias bei'ngso selected as to produce optimum detection efficiency.l VThe gain of the receiver is adjusted by the-gain' control potentiometer l l' so that the desired signalivolt-` agewill be substantially less'than thefbias" volt- "f age on grid `2l! of the triode detector tube $25.' 30

Potentiometer 4 If is adjusted to give the cathode 400i diode'23 `justsufficient positivebias to keep tube 23 non-conductive during the' receptionv of vsignal representing voltages.v When'V these adjustments have been made, detection of received signals will take placefin-tube `2li-"in a wellkn'ownmanner and the diode 23 will jre'- inain non-conductive." Also, as the grid Zd's not driven into its conductive-region 11o-addivtional `bias voltagewill buildup across the time constant Circuit 2li-2l. t

If `it is nowassumed that a noise pulse of considerably greater amplitude than the signal is picked upby the antenna system and transmitted through the receiver, thelirstpositive half cyclev will overcome the cathode bias of `the diode tube 2,3 and electrons will ilow from the 'cathode l0 to' a`node 22. 'This flow of velectrons has two functions. In theiirstplace,l a damping action results .due to the low impedancepresented byla conductive diode. This tends to' prevent grid '20 of the 'detector tube 25 from moving further in a4 positive direction, whichin turn prevents 11n-j due increase in the anode current of tube 25. In

''- the second place, electrons drawn from cathode Mirto diode plate 22-effect` arnegative `charge on grid 2li. This resultsin reducing the detecting eiciency of thistubeuntil the charge has leaked offthrough resistance 20. Ihave foundit advantageous to make resistancev 2B faboutv 100,000

ohms Vand capacity 2l about 0.0001 microfaradl Tests kthat I have made show that this time con- Vstaunt circuit'Rilf-Zi is important for satisfactory operation of my receiver. It is so regulated that t lthe charge on grid 2d will .leak oif by the time the noise pulse has ended. I believe that the beneficial results obtained when using this circuit can be attributed largely to lthe function of` `de-l creasing the detecting efliciency of tube 25 while the "noise transient persists.' n

The circuit described just above performs especially the function Aof limiting noise pulses that have vamplitudes, in excess' of the received carrier.

' Its operation may be observed. by closing switch Zitto4 Contact '29 and listening tothelsignalin telephone head set 3|. If the circuit is adjust- `detector vtube 25.

This tube 33 is shunted by' a- The foregoing description/identifies the `inte-- gral parts and explains the electrical connections ed as outlined above and a radio signal is being received, very satisfactory noise limitation will `be obtained. In practise the head set 3| may be replaced by any suitable type of loud speaker.

When carrier telegraph signals are to be received, switch 28 is closed to contact 30, thus connecting discharge tube 33 to the output of the Tube 33 `being -of the usual gaseous discharge type, will remain non-conductive until the voltage across it reaches about 120 volts.'` vIt will then become conductive and will remainfso until `the extinction voltage, usually about 1.05 volts, is reached. Resistance 21 and B `battery bi9 .are so related that in the absence of any signal the voltage drop across resistance 21 duetto the anode `current in tube 25 Willbe somewhat lower than the extinction voltage of `vvIf a carrier` wave is now tuned in, the average i impedance of-`tube25 drops, increasing `the voltage dro'pacross resistance 21`unti1 it reaches the amount *required to initiate the vgaseous discharge in tube 33. However, resistance -32` is relativelyhigh, being'of the order of 500,000 ohms,

and capacity 35 is about 0.002 microfarad'so that when tube 33 becomes conductive avoltage `drop develops across resistance 32 `Iand as soon as conductor ,35 has sufliciently discharged, the

voltage across tube 33 drops again below the eX- i ti'nction -value and fthe tubey becomes non-conductive. If the signal carrier isstill present, the

voltager across tube 33 will againbuild up until this tube begins to conduct and the cycle kwill be repeated.- The frequency of this recurring phenomenon is determined primarily by the values of resistance 32.y and capacity 35,' though it also depends to'some extent on the voltage drop available across resistance 21 and' on the impedance oi'ftheftelephone receiver 36. A proper selectionfoi these'factors will produce apleasing tone; substantially free of noise, in the' telephone receivers as long as the carrier `signal is being de'- modulated.

The resistance-32v and capacity V345 perforrn'anv additional function of importance in iiltering out audio frequencies which are present in the plate circuit of tube 25,' so that such audio frequencies Will not reach the telephone receiver 33. In consequencathere will be substantially complete silence in this telephone receiver when no tone is beingjgenerated by tubev 33.

When anlinterrupted carrier such as a telegraph signal is being received, .the tone generated by tube 33 will be heard during each dot and dash, 'and there will be complete silence between telegraphic characters. This complete intercharactersilence `will be obtained instead o f the usual noise interference. Where the noise interference is of lower amplitude than the received "carrier, the gain controlpotentiometer Itis adjusted so that the carrier signal will develop sufcient voltage drop across resistance 21 to trigger-off the tube 33, butv the noise voltages being ofy lower value` will not do so. When the itube 33l to? discharge:1.l rvSome: care mustifbe-"taken,

however, not to 'Smake capacity l34 toolargebr else the circuit maybe sloweddown to such'an extent vas -notfto' respond properly to telegraphic If a telegraph signal isi beinggreceived;` and properl adjustments have been madea tone 'generatedvby-tube33` will "belheard in thetelephone y Ireceiver lilli. Th'efduration of this tone'I will cor- 1`0 respond to the telegraphic dots and dashes-1 and the'I tone -'i will stop' during `the interhcharacter spaces:- Itis apparentthat noise 'of 'any amplit-v tude willnot be y.heard duringy the Adots `and dashes,

since audio'frequencyidisturbances cannot passv jfthe! iiltercomposedof resistance 32 and lcapac- A ity 34.f` fihe problem ytherefore `reduces to one Ot-keeping tlieftone generator from operating during the spaces-,between dots and dashes. Low

35 sufficiently to cause the` voltage across tuber 33 to reachthe ignition point and hence the tone generator'vvillnot be triggered oif.l Of course, if noise currentsof amplitude equal `to or .greater than that of lthe, 'received carrier, and off very. continuousnature were encountered, the tone generator would be triggered off and a falsesignal would result. It hasbeen `found in practice, 35 however, that interference of` this character `is rarely if ever encountered.

While I have illustrated my invention in connection with a radio receiver of the superheterodyne type, it willv be understood that its usefulf `ness is not limitedto such receivers. For example, rit may be usedwith a tuned radiofrequency receiver; and may also be used Ona wire carrier telegraph circuit. Various modifications and changes maybe made inthe system andmethod of operation described hereinabove without deby the appended claims. l

I claim: vr 1. .A" radio-telegraphic receiver Afor .receiving carrier wave telegraphic signals accompanied by` noise pulses of an intensity greater than and less than the carrier signal intensity and adapted to substantially reducethe audible interference normallyfcaused vby said pulses, comprising va negatively biased detector tube having an input and an output circuit, means included in said input circuit and actuated by excessive noise.

potential applied to said tone generator whereby the inter-character spaces of ysaid telegraphic signals will befsubstantially free of noise pulses.

2. A radio-telegraphic receiver for receiving carrier wave telegraphic signals accompanied by noise pulses of an intensity greater than and less I-Iigh amplitude noise will be limited byv If such noisepulses are of shortl parting from the scope of'my invention.- Some of the novel features of this invention are defined than the carrier signal intensity and adapted to substantially.-reducethe audible interference nor-- mally-fcaused by saidA pulses, comprising ia negatively' biaseddetector tube having arr-input and anoutput circuit, means lconnected tosaid input 'circuit and actuated by excessivenoise pulses for automatically increasingthe bia-s of saiddetector so that thedetecting 'eniciency of .-the'zt'ube is re'- "duced andthe .noise pulses loffgreater intensity thanjthe carrier substantially reduced,` va gaseous discharge tube .i connected Lto' the output circuit oft said detector tube, said gaseousdischargetube supplyingta pleasing tonel during the reception oi-*telegraphic signals andsupplying no tone fduring, inter-character spaces. f'

i y'3."In a radiorec'eiver, a vacuum tube detector forlthereception of high frequencysignal waves comprising an anode, cathode-'andl grid electrode, input and output circuits for said detector, biasing meansy connected to said cathode for establishing the normal point of operation of said tube in the. vicinity of the negative bendl of its grid voltage-plate current curve, a diode tube having its anode connected to thegrid of saiddetector tube,

adjustable means for biasing the cathode of said 'diode positive with respect toits anode` whereby said diode will remain inoperative during the reception of normal signals but will become conductive when excessive noise pulses are being Areceived, means connected to the grid of said detector comprising `a resistance shunted by a capacity for accumulating al negative'charge during. the reception of Aexcessive noise vpulses and negative bend of its grid voltage-.plate current characteristiccurve, an linput circuit for saidvdetector, a diode tube` and a resistance shunted by normally biased to operate in the vicinity of' the a capacity connected to the diode and to saidin-` ,put circuit for simultaneously dampingsaid circuit and altering the bias ofsaid detector during 'the reception of excessive'noise pulses, and means vforv preventing the damping of vsaidcir'cuit and alteration of said Vbiasy during/the reception of normal signal voltages. t

5. In a radio receiver, a signal wave detector; an" output circuitl for I said ldetector, liiltering means in said output vcircuit adaptedjto substantially remove audio frequency iiuct'uations'` from the output current of saiddetector, a tonegenerator in said output circuit adapted to be actuated by the rectified'outputof said vdetector and means for preventing the operation of 'said tone generator when the output of said detector falls below a predetermined minimum. v Y

6. In a radio receiver,v a signal wave detector, an output circuit for said detector, filtering means in said output circuit adapted to substantially remove audio frequency fluctuations from kthe output current of said detector, means for limit- `ing the rectified output current of said detector to an approximate predetermined maximum value!A a tone generator in said' output circuit adapted to be actuated by the rectied output of said detector and means for preventng the operl ation of said tone generator when the output of said .detector falls below apredetermined minimum.

7. Ina radio receiver-,a vacuum tube detector for the reception of high frequency signal waves comprising an anode, cathodeand grid electrode,l

inputandoutput circuits for said detector, biasin the vicinity of the negative bend of its grid Voltageeplate current curve, a diode tube having its anode connected to the grid of said detectorv tube, adjustable means for biasing the cathode of said diode positive with respect to itsl anode whereby said diodewill remain inoperative during the reception of normal signals but Will become conductive when excessive noise pulses are being received, means connected to the grid of said detector comp-rising a resistance shunted by a capacity for accumulating a negative charge during the reception of excessivev noise pulses, means for applying such additional negative bias to the gridl of said detector tube to decrease the sensitivity thereof during reception of noise impulses,

filtering rmeans i-nsaid output-circuitadapted' to substantially remove audio frequency fluctuations from the output current of saidv detector, a tone generator insaid outputcircuit adapted to be actuated bythe rectied output vofsaid detector, and means for preventing the operation of said tone generator ywhen the output vof said detector falls below a predetermined minimum.

8. A. radio telegraphio receiver as denedV in` Vmeans for automatically altering the detector bias andrprevents that means from altering the l bias during the reception of noise pulses oi an intensity less than the carrier signal intensity.

DANA A. GRIFFIN.V 

